The present invention relates to a lithium secondary battery using an ionic liquid as a nonaqueous electrolytic solution, and more particularly relates to a lithium secondary battery using a separator constituted by an electrically insulating porous inorganic membrane and a substrate.
A lithium secondary battery is a small-sized and light-weight chargeable battery having a large storage capacity per unit volume or unit weight, and thus is widely used for a mobile phone, a notebook computer, a personal digital assistant (PDA), a video camcorder, a digital still camera and the like, thereby becoming essential for a small-sized and light-weight portable device that consumes relatively large electric power.
In recent years, a middle-sized or large-sized lithium battery for an electric bicycle and an electric vehicle is being developed, and the development thereof is expected as a measure for reducing the environmental load.
As a nonaqueous solvent used in a nonaqueous electrolytic solution for a lithium secondary battery, a polar non-protonic organic solvent is used since it easily dissolves a lithium salt and is hard to suffer electrolysis, but the solvent has a considerably low flashing point and thus involves a severe problem in safety, such as ignition and explosion due to heat upon overcharge or short circuit. In particular, there is an increasing demand of development of a lithium secondary battery with large capacity and high output power associated with reduction in size and weight of an electric device and development of an electric vehicle, the problem in safety is becoming an important problem to be solved.
Under the circumstances, the use of an ionic liquid, which is an incombustible compound, as a nonaqueous electrolytic solution of a lithium secondary battery is variously studied. As the use of an ionic liquid as a nonaqueous electrolyte, for example, the use of an ionic liquid, which contains a bis(fluorosulfonyl) imide anion as an anionic component, as a solvent is disclosed (Patent Document 1).
As a separator, a separator for a lithium high energy battery containing a flat flexible substrate having a large number of openings and a porous inorganic membrane is disclosed (Patent Document 2).
Patent Document 1: U.S. Pat. No. 6,365,301
Patent Document 2: JP-T-2005-536857